BoilerFaultCodesViessmann 4C Fault Code
The Viessmann 4C fault code means: your boiler has lost communication with a sensor that monitors the water temperature for your third heating zone, which will prevent that specific area from heating correctly.. It appears on 2 Viessmann models in our database.
Technical description: Lead break, flow temperature sensor, heating circuit 3 (with mixer)
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Viessmann models that show the 4C fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 4C fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026